How do I view someones IP who has made 0 post and hasnt been on for a while?
You will only be able to view the IP address which they have registered with. So, if the user has a dynamic IP address, it won't be accurate.
Anyways, to find the IP address of a specific user, go into your Admin CP > Users & Groups > Search for X user > With the results, click the "Options" button, and select "Show IP Addresses".
You can see the current IP in the Who's Online list, only if they're online - obviously.